Method and apparatus for improving quality of multimedia streaming service

ABSTRACT

In an apparatus for providing a multimedia streaming service, a server transmits a Media Presentation Description (MPD) including information about media data to a client, receives a partial request message requesting a part of media data having a range according to a defined range in the MPD, and transmits to the client a segment having the range in response to the partial request message. The segment includes at least one fragment, segment index information indicating the position of the at least one fragment in the segment, and fragment index information including information indicating the position of each of a plurality of samples included in the at least one fragment and information about a total size of the samples.

PRIORITY

This application is a Continuation of, and claims priority under 35U.S.C. §120 to, U.S. patent application Ser. No. 13/187,113, filed Jul.20, 2011, now allowed, which claims priority under 35 U.S.C. §119(a) toKorean Patent Applications filed in the Korean Intellectual PropertyOffice on Jul. 20, 2010 and assigned Serial No. 10-2010-0070194, on Aug.19, 2010 and assigned Serial No. 10-2010-0080553, on Oct. 15, 2010 andassigned Serial No. 10-2010-0101121, on Jan. 24, 2011 and assignedSerial No. 10-2011-0007075, and on Jan. 25, 2011 and assigned Serial No.10-2011-0007236, respectively, the contents of all of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to Hypertext Transfer Protocol(HTTP)/Transmission Control Protocol (TCP)-based multimedia service, andmore particularly, to an apparatus and method for ensuring servicequality in HTTP/TCP-based multimedia service.

2. Description of the Related Art

In HTTP-based multimedia service, data is transmitted and receivedbetween a client and a server by exchanging a content request (HTTP GET)with a response (HTTP Response) to the content request. When the clientinitially accesses the server, the server transmits a serviceablecontent list and a Media Presentation Description (MPD) for mediacontent to the client. The MPD describes information required for theclient to receive the media content, such as the type of the mediacontent, the average bit rate of the media content, and the UniformResource Identifiers (URIs) or Uniform Resource Locators (URLs) ofcontent Segments covering a time unit. The client repeatedly requestsnecessary content based on the MPD.

Clients differ as to their terminals and network situations. To meetservice requirements of clients of various terminals in variousnetworks, the server may have coded streams with different qualitylevels for the same content. Therefore, a client may request a streamwith an appropriate quality to the server according to its terminal ornetwork state, thereby enabling a seamless service.

The MPD segments a media stream on a time unit basis. Thus, each time aclient requests media content, the client selects one of stream segmentsin the same time zone according to its situation and transmits an HTTPGET for the selected stream segment to the server. In response to theclient request, the server transmits the stream segment together with aresponse message header (with status code 200 OK).

If the response message is too large, the server transmits the responsemessage separately in one or more TCP packets. Upon successful receiptof all TCP packets, the client reconstructs the original HTTP responsemessage with the TPC packets. TCP adopts Automatic Repeat reQuest (ARQ)to ensure the reliability of data transmission and reception. Accordingto ARQ, when a transmitter receives a response message indicatingdetection of an error in received data, that is, a NegativeACKnowledgment (NACK) message from a receiver, or fails to receive anyresponse message within a predetermined time from the receiver, thetransmitter automatically retransmits data to the receiver. Therefore,transmission reliability can be ensured for every TCP-based service,such as an HTTP or File Transfer Protocol (FTP) service.

If any of the TCP packets is lost or erroneous, the TCP packet isretransmitted until it is successfully received. In this manner,TCP-based services such as HTTP or FTP services achieve transmissionreliability.

However, when a TCP packet is lost due to congestion or interferenceduring data transmission or retransmission, unreliability occursfrequently at the TCP layer because of errors or a poor channel state,an end-to-end transmission delay becomes excessive.

Moreover, if the network state is poor and the size of transmission dataat the HTTP layer is large, the end-to-end transmission delay becomesexcessive and compromises service quality for a delay-sensitive servicesuch as multimedia service.

FIG. 1 illustrates a data transmission and reception operation at theHTTP layer (in the upper diagram) and a data transmission and receptionoperation at the TCP layer (in the lower diagram), according to theprior art.

In the upper diagram of FIG. 1, a server responds to a request receivedfrom a client at the HTTP layer. Only when the client completelyreceives a response message (with status code 200 OK) from the server,may the client process the response message.

In the lower diagram of FIG. 1, the server transmits the responsemessage separately in TCP packets to the client at the TCP layer. Upongeneration of an error or data loss during transmission, an erroneous orlost TCP packet is retransmitted. As this operation is repeated untilall TCP packets are successfully transmitted, a transmission delay iscaused. In addition, when data loss occurs, the resulting decreasedtransmission rate also leads to a transmission delay.

In a conventional HTTP-based media streaming service, a stream issegmented on a time basis and a stream segment requested by a client isdelivered to the client. Therefore, it becomes difficult to reduce aninitial buffering delay.

TCP also adopts a slow start method for flow control. According to theslow start method, upon generation of data loss during data transmissionat a gradually increased bit rate, the bit rate is reduced significantlyand then data is transmitted at the low bit rate. Since an available bitrate cannot be fully utilized in wireless communication experiencing theaforementioned data loss, the slow start method significantly decreasessystem efficiency.

SUMMARY OF THE INVENTION

An aspect of embodiments of the present invention is to address at leastthe problems and/or disadvantages and to provide at least the advantagesdescribed below. Accordingly, an aspect of the present invention is toprovide a method and apparatus for providing an HTTP-based multimediastreaming service to prevent degradation of service quality caused byend-to-end transmission delay.

Another aspect of the present invention is to provide a method andapparatus for providing an HTTP-based multimedia streaming service toreduce a transmission delay and increase resource utilizationefficiency.

A further aspect of the present invention is to provide a method andapparatus for providing an HTTP-based multimedia streaming service toensure service quality through reduction of a possible transmissiondelay and full utilization of an available bit rate.

In accordance with one aspect of the present invention, there isprovided a method for providing a multimedia streaming service, in whicha server transmits a Media Presentation Description (MPD) includinginformation about media data to a client, receives from the client apartial request message requesting a part of media data having a rangebased on a range defined in the MPD, and transmits to the client asegment having the range in response to the partial request message. Thesegment includes at least one fragment, segment index informationindicating the position of the at least one fragment in the segment, andfragment index information including information indicating the positionof each of a plurality of samples included in the at least one fragmentand information about a total size of the samples.

In accordance with another aspect of the present invention, there isprovided a method for receiving a multimedia streaming service, in whicha client receives an MPD including information about media data from aserver, detects segment information in the received MPD, transmits tothe server a partial request message requesting a part of media datahaving a range based on a range defined in the MPD, and receives fromthe server a segment having the range in response to the partial requestmessage. The segment includes at least one fragment, segment indexinformation indicating the position of the at least one fragment in thesegment, and fragment index information including information indicatingthe position of each of a plurality of samples included in the at leastone fragment and information about a total size of the samples.

In accordance with yet another aspect of the present invention, there isprovided a server apparatus for providing a multimedia streamingservice, including a transmitter configured to transmit an MPD includinginformation about media data to a client, and a transceiver configuredto receive from the client a partial request message requesting a partof media data having a range based on a range defined in the MPD, and totransmit to the client a segment having the range in response to thepartial request message. The segment includes at least one fragment,segment index information indicating the position of the at least onefragment in the segment, and fragment index information includinginformation indicating the position of each of a plurality of samplesincluded in the at least one fragment and information about a total sizeof the samples.

In accordance with a further still aspect of the present invention,there is provided a client apparatus for receiving a multimediastreaming service, including a receiver configured to receive an MPDincluding information about media data from a server, a data requesterconfigured to detect segment information in the received MPD and totransmit to the server a partial request message requesting a part ofmedia data having a range according to a range defined in the MPD, and adata receiver configured to receive from the server a segment having therange in response to the partial request message. The segment includesat least one fragment, segment index information indicating the positionof the at least one fragment in the segment, and fragment indexinformation including information indicating the position of each of aplurality of samples included in the at least one fragment andinformation about a total size of the samples.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of certainembodiments of the present invention will be more apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a conventional data transmission and receptionoperation at an HTTP layer (in the upper diagram) and a datatransmission and reception operation at a Transmission Control Protocol(TCP) (in the lower diagram);

FIG. 2 illustrates a Moving Picture Experts Group-4 (MPEG-4)/AdvancedVideo Coding Network Abstraction Layer (AVC NAL) header, to whichembodiments of the present invention are applied;

FIG. 3 illustrates a client timeout-based transmission and receptionoperation, according to an embodiment of the present invention;

FIG. 4 illustrates a transmission and reception operation based onclient timeout and HTTP partial transmission, according to an embodimentof the present invention;

FIG. 5 illustrates a client buffer model, according to an embodiment ofthe present invention;

FIG. 6 illustrates a transmission delay situation, according to anembodiment of the present invention;

FIG. 7 illustrates an application layer buffer and an initial delay,according to an embodiment of the present invention;

FIG. 8 illustrates a method for reducing a buffering delay by changing atarget value for a steady buffer state, according to an embodiment ofthe present invention;

FIG. 9 illustrates a method for reducing a buffering delay based onintra frames, according to an embodiment of the present invention;

FIG. 10 illustrates a media index structure for MPEG and 3^(rd)Generation Partnership Project (3GPP) HTTP streaming according to anembodiment of the present invention;

FIG. 11 illustrates a Segment structure according to an embodiment ofthe present invention; and

FIG. 12 illustrates a stream structure produced through sampledisplacement according to an embodiment of the present invention.

Throughout the drawings, the same drawing reference numerals will beunderstood to refer to the same elements, features and structures.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. In the followingdescription and the accompanying drawings, well-known functions andstructures will not be described if they unnecessarily obscure thesubject matter of the present invention. Further, a detailed descriptionof known functions and configurations incorporated herein will beomitted for the sake of clarity and conciseness.

The present application discloses the following three methods to reducea transmission delay and thus to ensure service quality in an HTTP-basedmultimedia streaming service.

One of the transmission delay reduction methods is timeout-based datatransmission and reception. Since the multimedia service isdelay-sensitive, repeated retransmissions intended to ensure reliabilityonly degrade service quality. Accordingly, a server, a client or a proxysets a timeout, taking into account the status of a client buffer andthe situation of the network, and only data transmitted within thetimeout is processed, thereby reducing an end-to-end transmission delayfor a user in a poor network situation. Accordingly, the use of HTTPpartial transmission and timeout can lead to a decreased transmissiondelay, reduce an initial presentation delay and a zapping delay causedby channel zapping and random access, and provide reliable servicequalities.

Another transmission delay reduction method is to prevent a slow startthrough ACKnowledgment (ACK) spoofing, in spite of data loss on a radiochannel and thus to reduce a transmission delay and efficiently utilizean available bit rate.

A third method for transmission delay reduction is to slowly starttransmission with low-quality data, using a preview channel or Segment,so as to reduce an initial buffering delay and a channel zapping delay.

A detailed description will now be given of a delay reduction methodaccording to an embodiment of the present invention.

An MPD for HTTP partial transmission contains information about prioritylevels of media content and range information for each priority level.For example, when an MPEG-4/AVC-encoded video is serviced, a stream issegmented on a time unit basis (e.g. 2 seconds) and information aboutthe addresses of the stream segments is written in an MPD, as inconventional HTTP-based media streaming. Therefore, a client mayconventionally request and receive a service only on the basis of astream time unit specified by the MPD.

In this context, the embodiment of the present invention seeks tosatisfy various clients' demands by writing media priority informationand stream range information reconstructed using the priorityinformation (offsets or positions in a stream file) in an MPD.

FIG. 2 illustrates an MPEG-4/AVC NAL header, to which an embodiment ofthe present invention is applied.

A stream is reconstructed to have the same NAL Reference Index (NRI)value, for example, using an NRI field in the MPEG-4/AVC NACL header andthe NRI value and range information about the stream is written in anMPD. A client requests a part of content (e.g. a preview) by requestingpartial transmission of data including only intra frames (HTTP PartialGET) using the NRI field. In this manner, a part (e.g. a preview) of thecontent may be displayed to a user.

Conventionally, ACK spoofing is used at the TCP layer in order to solvethe transmission delay problem. However, the TCP needs modifying to useACK spoofing at the TCP layer. Accordingly, the present applicationdiscloses timeout at an application layer, which may be used by aserver, a client, or a proxy in a network to reduce a transmission delaythat may occur due to TCP-layer ARQ.

FIG. 3 illustrates a client timeout-based transmission and receptionoperation, according to an embodiment of the present invention.

Referring to FIG. 3, an HTTP streaming client requests necessary data(HTTP GET) and then activates a timer (or counter) for a timeout event.Upon receipt of a response message to the request from the server beforethe timeout event occurs, the HTTP streaming client processes theresponse message as normally received. On the other hand, upongeneration of the timeout event before the response message arrives atthe client, the client takes the next action irrespective of whether theresponse message has been received.

FIG. 4 illustrates a transmission and reception operation based onclient timeout and HTTP partial transmission, according to an embodimentof the present invention.

A server-based timeout method and a proxy-based timeout method are thesame in terms of their processes except for entities responsible fordetermining a timeout. A timeout determining procedure is most importantin a timeout-based method. In this embodiment of the present invention,a timeout is determined based on the buffer state of a client andcontent quality is additionally adapted for the client according to thebuffer state of the client. An example of a client buffer model will bedescribed below.

As stated before, Periods or ranges of a transmittable media data fileare specified in an MPD in a file structure designed to use the abovemethod. The server or client receives the information and performscontent quality adaptation by a partial transmission and unit structuretransmission scheme, referring to the specified Periods and ranges.

FIG. 5 illustrates a client buffer model, according to an embodiment ofthe present invention.

A client buffer may be divided largely into an input unit for receivingdata that has normally arrived (within a timeout), an output unit foroutputting buffered data for decoding, and a buffer controller forperforming service quality adaptation and determining a timeoutaccording to a current buffer length.

A data input rate is determined for the input unit according to aservice request of the client and a network situation. A drain rate isdetermined for the output unit according to the quality of currentlyoutput content. The buffer controller first determines a buffer length.Specifically, the buffer controller determines the total buffer lengthof the client by summing the buffer requirements of a buffer needed forservice content, a buffer needed due to a network delay, and aretransmission buffer used to cope with a retransmission delay over awired/wireless interface.

In FIG. 5, the service quality level and timeout of the next service areadjusted by comparing two decision points or levels inside a buffer witha current buffer length. For instance, the buffer controller may operateas follows.

If the amount of data buffered in the buffer, that is, a buffer lengthis larger than an upper decision point, buffer overflow is expected.Thus, a long timeout is set to thereby increase transmissionreliability, and a time gap is set before a request of the next Segment.If the buffer length is smaller than a lower decision point, the servicelevel and the timeout are decreased due to expected buffer underflow.

However, when a service level and a timeout are determined using onlydecision points and a current buffer length, the resulting frequentchanges may decrease a desired quality. Introduction of a counter maydecrease the frequent service level and timeout changes. In addition,although some data may be lost due to the timeout, the lost data can berecovered by a channel coding method.

1) Performance Improvement by Allowing Partial Reception

(k-n) parity symbols are added to a Segment with k symbols andtransmission of the total n symbols is attempted. If any k symbols outof the n symbols are received, the transmission is discontinued usingthe above-described timeout method, thereby reducing a transmissiontime. That is, the transmission time is shortened by terminating thetransmission when any k symbols out of the n transmitted symbols (n>k)are received, rather than when all of k transmitted symbols arereceived. If the (n−k) parity symbols are created using a Reed Solomon(RS) code, reception of k symbols out of the n transmitted symbol issufficient for data recovery. However, with a Raptor code, more than ksymbols need to be received in order to increase a recovery completionprobability. When the RS code is used, the number of received symbols ismonitored and a timeout is declared upon receipt of k symbols. When theRaptor code is used, Raptor decoding is performed simultaneously withsymbol reception. Upon completion of the Raptor decoding, a timeout isdeclared, which is more efficient.

2) Performance Improvement Based on ACK Spoofing

FIG. 6 illustrates a transmission delay situation. Conventionally, TCPadopts slow start for flow control, as illustrated in (b) of FIG. 6.According to the slow start method, a bit rate is gradually increased.When data loss occurs, the bit rate is significantly dropped and thenthe slow start re-occurs, for flow and congestion control. As thisoperation is repeated, the bit rate fluctuates as illustrated in (b) ofFIG. 6. As a result, an available bit rate is not fully utilized,causing a transmission delay. Particularly, wireless communicationsuffers a longer transmission delay because transmission starts slowlyat a bit rate that has not increased sufficiently due to packet losscaused by fading-incurred bit errors or collision on a channel.Accordingly, embodiments of the present invention reduce the number ofslow starts through ACK spoofing, in case of packet loss caused byfading or collision on a channel. As partial reception is allowedaccording to the afore-described ‘partial reception method’, ACKspoofing enables full utilization of an available bit rate and completereception of an intended Segment.

3) Performance Improvement by Preview

According to an embodiment of the present invention, a buffering delaysuch as an initial delay and a channel zapping delay can be reduced inHTTP-based multimedia streaming.

FIG. 7 illustrates an application layer buffer and an initial delay,according to an embodiment of the present invention.

Referring to FIG. 7, an initial buffering delay is determined, takinginto account de-jittering and de-interleaving. In case of Video OnDemand (VOD), the initial buffering delay is typically 5 to 10 seconds.After the buffering delay, a steady buffer state is maintained and thusinterrupt-free playback is possible. A de-jittering delay is determinedaccording to a change in an end-to-end transmission rate and a change ina bit rate required for content.

Let the de-jittering and de-interleaving delay be denoted by T_(D) andthe maximum rate of a media stream be denoted by R_(max). Then, aninitial buffer length B_(max)=R_(max)*T_(D). That is, when as much dataas B_(max)=R_(max)*T_(D) is buffered in a receiver buffer, a client maystart to play back the media stream. Although a signaling delay may notbe reduced, the buffering delay can be reduced by decreasing the maximumrate of a media stream, R_(max), which means decreasing a target valuefor the steady buffer state.

FIG. 8 illustrates a method for reducing a buffering delay by changing atarget value for the steady buffer state, according to an embodiment ofthe present invention.

For example, a video stream is encoded by layered coding and a baselayer stream with a maximum bit rate r_(max)(<R_(max)) and anenhancement layer are separately stored. Only the base layer isinitially transmitted until b_(max)=r_(max)*T_(D). In this manner, thebuffering delay may be reduced. Since 10r_(max)=R_(max) on the whole,the buffering delay is decreased to about 1/10 in this method. Whenbuffering reaches B_(max), a video is played back with normal quality.This method is also applicable to a layered coding scheme and othergeneral coding schemes, but is inefficient in a poor network situation.

Second, if even though the steady buffer state has not been reached,playback starts immediately when reproducible information arrives, thetime taken for initial presentation may be reduced.

FIG. 9 illustrates a method for reducing a buffering delay based onintra frames, according to an embodiment of the present invention.

Referring to FIG. 9, upon arrival of at least one intra frame, playbackimmediately starts, thereby shortening the time taken for initialpresentation. Therefore, the initial buffering delay may be reduced bymeans of an MPD containing priority information according to anembodiment of the present invention and HTTP partial transmission.

If the Group Of Picture (GOP) period of a base layer is 1/10 of a TD,the buffering delay may be reduced to 1/10 according to this method.Encoding of the base layer in such a manner that the intra period of thebase layer is smaller than the intra period of the enhancement layerwill be helpful. An initial presentation of the base layer maysufficiently notify a user of what is broadcast on this channel. Amarker is needed to indicate arrival of an intra frame of the base layerto the system.

The two methods according to the two embodiments of the presentinvention are applicable to both cases of starting content and channelswitching. According to these methods, an initial delay and channelzapping delay of 5 to 10 seconds at present may be decreased to or belowabout 200 ms.

The present invention can be applied to HTTP streaming, thestandardization of which is being developed by the MPEG-Dynamic AdaptiveHTTP Streaming (DASH) and 3^(rd) Generation Partnership Project (3GPP).The MPEG-DASH and 3GPP defines an MPD for HTTP streaming. An HTTPstreaming client may index one or more Periods, Representations, andSegments based on the MPD of media content, as illustrated in FIG. 10.

Specifically, FIG. 10 illustrates a media index structure for MPEG and3GPP HTTP streaming according to an embodiment of the present invention.

An MPD extension according to an embodiment of the present invention mayextend SegmentInfo of an existing MPD, as illustrated in Table 1.

TABLE 1 type A CM Indicates the type of a corresponding Must bebyte-range of the Segment, if a range present if exists for the Segment.In case of the “range” SegmentIndexBox, the type is sidx(s). attributeThe type is set to 0, 1, . . . , N for is present Priority and to timefor sub-time. If Priority is ‘0’, this is the highest priority. If asingle Segment is divided into one or more ranges, the type of eachrange is distinguished by;. duration A CM Defines a duration for eachrange. If a Must be single Segment is divided into one or present ifmore ranges, the duration of each range the “type” is distinguished by;.attribute is “time”

In accordance with the embodiment of the present invention, the aboveSegmentInfo extension enables an effective operation for each of thefollowing user cases.

(Case 1) “Peter watched a soccer match involving his favorite team. Thegoal in the 64th minute made him so excited that he wants to watch itover and over again. He finds a recording of the game on the homepage ofhis local TV station and requests the segment containing the goal.”

In general, an MPD and index information contained in a segment index(sidx) box enables a DASH client of Peter to seek Movie Fragment(s)(moof(s)) within a Segment corresponding to the 64^(th) minute. Themoof(s) that provide (provides) a Representation of the thrilling goalmay be directly requested by an HTTP partial request.

In comparison, the following two methods are available according toembodiments of the present invention.

To access the moof(s) corresponding to the 64^(th) minute that Peterwants to repeatedly watch in the current WD, the following procedure isneeded.

A Segment nearest to the intended moof(s) is indexed based onSegmentInfo of the MPD and then transmission of sidx(s) is requested toindex the moof(s) corresponding to the intended time within the Segment.

If the Segment has a single sidx, the sidx(s) transmission is requestedby one HTTP partial request without sidx size information. If theSegment has multiple sidxs, the sidx(s) transmission is requested by twoor more HTTP partial requests without sidx size information.

Subsequently, the intended moof(s) is(are) requested based on thedownloaded sidx(s).

When the disclosed MPD extension (range with type=“sidx(s)”) is used,the following procedure is needed.

A Segment nearest to the intended moof(s) is indexed based onSegmentInfo of the MPD. Then, to index the moof(s) corresponding to theintended time within the Segment, transmission of sidx(s) is requested.

If the Segment has a single sidx, the sidx(s) transmission is requestedby one HTTP partial request with sidx size information. If the Segmenthas multiple sidxs, the sidx(s) transmission is also requested by oneHTTP partial request with sidx size information.

Subsequently, the intended moof(s) is(are) requested based on thedownloaded sidx(s).

As described above, the use of the disclosed MPD extension may reduceservice-client interactions needed for a user to request desiredmoof(s). In addition, sidx(s) of an accurate size may be requested by anHTTP partial request due to the presence of sidx(s) size information.

(Case 2) “Paul can't attend a concert given by his favorite band, butfinds out that there will be a live stream of it on the band's homepage.He starts watching, but later on his friend comes by and wants to seeit, too. Paul stops the stream and they watch it from beginning to end.”

In general, an MPD for live streams describes the sequence of Segmentsincluding media content. The Segments remain valid at the same URL. ThetimeShiftBufferDepth attribute of the MPD indicates the time window ofeffectively (inexplicitly in this case) available Segments. A DASHclient of Paul can easily request a Segment corresponding to the startof the Media Presentation and thus play back the Media Presentation fromthe start.

In accordance with the embodiment of the present invention, a Segmentnearest to intended moof(s) is based on SegmentInfo of the MPD. Then,transmission of sidx(s) is requested to index the moof(s) correspondingto an intended time within the Segment.

If the Segment has a single sidx, the sidx(s) transmission is requestedby one HTTP partial request without sidx size information. If theSegment has multiple sidxs, the sidx(s) transmission is requested by twoor more HTTP partial requests without sidx size information.

Subsequently, the intended moof(s) is(are) requested based on thedownloaded sidx(s).

(Case 3) “Mary wants to watch a stream of a TV series, but wants to skipthe opening credits. She requests the stream starting from minute 3.”

In general, an MPD and index information contained in a segment indexbox enables a DASH client of Mary to seek moof(s) corresponding to the3^(rd) minute within a Segment. The moof(s) that provide (provides) aRepresentation starting at the 3^(rd) minute may be directly requestedby an HTTP partial request.

In comparison, the following two methods are available according toembodiments of the present invention.

To access the moof(s) corresponding to the 3^(rd) minute that Mary wantsto watch in the current WD, the following procedure is needed.

A Segment nearest to the intended moof(s) is indexed based onSegmentInfo of an MPD, and then transmission of sidx(s) is requested toindex the moof(s) corresponding to the intended time within the Segment.

If the Segment has a single sidx, the sidx(s) transmission is requestedby one HTTP partial request without sidx size information. If theSegment has multiple sidxs, the sidx(s) transmission is requested by twoor more HTTP partial requests without sidx size information.

Subsequently, the intended moof(s) is(are) requested based on thedownloaded sidx(s).

Meanwhile, when the disclosed MPD extension (range with type=“sidx(s)”)is used, the following procedure is needed.

A Segment nearest to the intended moof(s) is indexed based onSegmentInfo of an MPD. Then, to index the moof(s) corresponding to theintended time within the Segment, transmission of sidx(s) is requested.

If the Segment has a single sidx, the sidx(s) transmission is requestedby one HTTP partial request with sidx size information. If the Segmenthas multiple sidxs, the sidx(s) transmission is also requested by oneHTTP partial request with sidx size information.

Subsequently, the intended moof(s) is(are) requested based on thedownloaded sidx(s).

As described above, the use of the disclosed MPD extension may reduceservice-client interactions needed for a user to request a desiredmoof(s). In addition, sidx(s) of an accurate size may be requested by anHTTP partial request due to the presence of sidx(s) size information.

(Case 4) “Peter requests a new movie that was published recently and isnot stored in the selected server. The movie file is efficientlydistributed to the server to facilitate the request.”

In general, this corresponds to a standard mode for HTTP caching inservers or Content Distribution Networks (CDNs). An attachment pointinitially selects a server according to Peter's network situation by apotential CDN load-balancing algorithm. A DASH client of Peter transmitsan HTTP request for moof(s). Then the HTTP request is subjected to astandard HTTP cache procedure and not present in an HTTP cacherequesting a Segment in an up-stream server. Eventually, the HTTPrequest reaches an origin server that ensures validity of a Segment.

In accordance with embodiments of the present invention, however, torapidly respond to the user request, an origin server selectively usesan HTTP caching server or a CDN regarding a user's high-prioritydownload request (e.g. type=“0”) or a user's frequent request (e.g.type=“sidx(s)”). Thus, efficiency can be increased in terms of networkbandwidth or memory space.

(Case 5) “Frank is watching a stream with a mobile device. In thebeginning, he is moving a lot causing high network throughputfluctuations. When he finds a cafe, he sits down and watches the rest ofthe stream in relatively stable network conditions using the cafe'sWLAN.”

In general, when continuous streaming is insufficient in a currentRepresentation (at a current bit rate) or streaming is sufficient in ahigh-quality Representation, a transmission rate makes a DASH client ofFrank determine to continuously monitor an input data rate. At thismoment, index information in an MPD is used to detect an appropriateSegment in an alternative Representation for switching. A segment indexbox of the Segment is used to detect a switching point in the form of aRandom Access Point (RAP) not later than the end presentation time ofthe latest requested data in the current Representation. The DASH clientstarts to request data of the new Representation and provides the mediacontent of the old Representation to a media player, for a subsequentseamless playback.

When Frank enters the WLAN area, the DASH client detects sufficientlyfast arrival of data to support streaming of the high-qualityRepresentation. The switching procedure is repeated and high-qualityRepresentation ensues.

In order to provide a seamless service in a mobile environmentexperiencing substantial network throughput fluctuations, representationswitching should be fast against throughput changes. A client maydetermine an appropriate Representation matching a current throughputand an appropriate Segment matching a switching time, based on an MPD.However, the client should download sidx(s) to find out an appropriateRAP in the Segment.

Accordingly, at least one HTTP partial request is necessary according toan arrangement of sidx(s) in a Segment according to this embodiment ofthe present invention.

Meanwhile, in case of the MPD extension, all sidx(s) of the Segment canbe downloaded by one HTTP partial request, based on a range attribute oftype=“sidx(s)” in SegmentInfo of the MPD.

If ranges(s), a type, and duration(s) are defined according to temporallevels or a frame type, switching is possible without sidx(s), usingranges(s) with the highest priority (e.g. type=“0”; I frame) and theirduration(s).

To enable fast Representation switching according to a throughput changein a mobile network environment experiencing serious throughputfluctuations, server-client interactions should be reduced and abandwidth-efficient sidx(s) request should be issued. In this context,the use of the MPD extension enables Representation switching basedsolely on the MPD, with a reduced number of interactions for sidx(s)download or without an additional interaction through a Priority type,and a duration.

(Case 6) “Tom has a DASH-ready mobile phone and wants to watch moviecontent that is available at 3 bit rates. While watching, Tom boards acrowded tram and available bandwidth of the tram is less than the 3 bitrates. Tom can continue to watch the movie.”

User experience expected from this user case is not apparent. If a validbandwidth is below the lowest available bit rate, Tom may continue towatch the movie at the lowest of the three bit rates at a normal framerate.

Meanwhile, in embodiments according to the present invention, if aclient transmits a segment wise HTTP request in a network environment inwhich the throughput of the client is lower than a Representation withthe smallest bandwidth, the client pauses the playback due to bufferunderflow and performs buffering. When receiving sidx(s) information,the client plays back the content at a low frame rate, instead ofpausing the playback due to buffer underflow, by requesting only moof(s)independently decodable in a Segment using an HTTP partial request.

Further in embodiments according to the present invention, with the MPDextension, a client may transmit an HTTP partial request for onlymoof(s) independently decodable in a Segment, based on sidx(s)information and may play back the moof(s), as in the above embodiment.

In the presence of Priorities (e.g. type=“0, 1, . . . , N”) and rangesaccording to temporal levels of a hierarchical prediction structure inSegmentInfo of an MPD, a client transmits an HTTP partial request onlyfor a high-priority range and plays it back.

If a range and a type with “sidx(s)” in the MPD extension are used,server-client interactions needed to receive all sidx(s) can be reduced.

Media priority information and stream range information according to anembodiment of the present invention are described in an MPD, for thefollowing use. For instance, MPEG-DASH and 3GPP Adaptive HTTP Streaming(AHS) under standardization divides each media stream into predeterminedunits (i.e. moofs) and one or more moofs are grouped into a Segment in atime order. A single moof may be composed of a plurality of groups (e.g.GOPs) of media samples (e.g. video frames or MPEG/AVC NAL units). Toprovide index information about moofs in a single Segment, DASH uses asegment index box (sidx).

FIG. 11 illustrates a Segment structure according to an embodiment ofthe present invention.

Referring to FIG. 11, S denotes sidx and f1 to f6 denote moofs (i.e.fragments). sidx specifies the position of each moof box in a file so asto enable moofwise access, when needed. Table 2 illustrates the syntaxof sidx of MPEG-DASH and 3GPP AHS, by way of example.

TABLE 2 aligned(8) class SegmentIndexBox extends FullBox(‘sidx’,version, 0) {   unsigned int(32) reference_track_ID;   unsigned int(16)track_count;   unsigned int(16) reference_count;   for (i=1; i<=track_count; i++)   {     unsigned int(32)track_ID;     if (version==0)    {       unsigned int(32)decoding_time;     } else     {      unsigned int(64)decoding_time;     }   }   for(i=1; i <=reference_count; i++)   {     bit (1)        reference_type;    unsigned int(32)reference_offset;     bit(1)        contains_RAP;    unsigned int(31)RAP_delta_time;   } }

When a Segment has the configuration illustrated in FIG. 11, each moofincludes frames with different temporal levels (e.g. I, P and B frames).When a Trick and Random Access (TRA) situation occurs upon user request,only a specific sample group (e.g. subfragments) such as an I framegroup) in each moof is played back. To support this operation, indexinformation enabling access to a certain group of samples in the moofsneeds to be added.

Methods for supporting access to a specific sample group are provided inthe following application examples.

Application Example 1 (sidx_extension)

Table 3 illustrates a syntax that describes an sidx_extension-basedmethod, according to an embodiment of the present invention.

TABLE 3 aligned(8) class SegmentIndexBox extends FullBox(‘sidx’,version, 0) {  unsigned int(32) reference_track_ID;  unsigned int(16)track_count;  unsigned int(16) reference_count;  for (i=1; i<=track_count; i++)  {   unsigned int(32)track_ID;   if (version==0)   {    unsigned int(32) decoding_time;   } else   {     unsigned int(64)decoding_time;   }  }  for(i=1; i <= reference_count; i++)  {   bit(1) reference_type;   unsigned int(31)reference_offset;  bit(1) contains_level; //indication flag for level, added   unsignedint(31)subsegment_duration; //reduce one bit for contains_level,modified   bit(1) contains_RAP;   unsigned int(31)RAP_delta_time;     if(contains_level)  //added     {     unsigned int(8) assemble_type;  //define assemble type, add     unsigned int(16) level_count; //numberof level(e.g. temporal id), added     for(i=1; i <= level_count;i++) //added      {       unsigned int(8) level; //level (e.g. temporalid level) define, added       bit(1) reserved_bit;  //reserved      unsigned int(31) level_offset; //offest, added      if(assemble_type == 0x01 || assemble_type == 0x03)       {       unsigned int(16) offset_count //define no. of samples in level,added        for(j=1; j <= offset_count; j++) //added         {         unsigned int(32) offset; //offset from level_offset, added         unsigned int(32) size; //size of each sample in level, added        }       }      }     }  } }

Parameters in Table 3 have the following meanings

contains_level: a flag bit indicating whether subfragmentwise indexinformation is included; and

assemble_type: an indicator indicating a media sample arrangement methodof each moof.

Table 4 illustrates a use example of assemble_type.

TABLE 4 assemble_type Definition 0x00 Sample group box(e.g. tele box)0x01 Assembled by equal temporal_level(e.g. IIPPBB . . .) 0x02Stereoscopic plain(e.g. LRLRLR . . .) 0x03 Assembled by view(e.g. LLLRRR. . .) 0x04 SVC plain(e.g. BEBEBE . . .) 0x05 Assembled by equalscalability type(e.g. BBBEEE . . .) 0x06~ Reserved level_count: thetotal number of levels in a fragment; level: defines each level. A lowervalue indicates a higher priority; level_offset: position information ofeach level; offset_count: an offset count to support samplewise access,when needed; offset: the position of a sample; size: the size of asample; reserved_bit: a reserved bit for extension.

The above sidx_extension enables a user to directly access a samplegroup having a specific level and thus a trick mode, Picture in Picture(PIP), and rate adaptation to a network environment can be moreeffectively supported.

The following example presents a method for providing index informationabout a subfragment according to a priority in a moof including one ormore priorities (i.e. levels) such as a Scalable Video Coding (SVC)moof, a stereoscopic moof, and a Multiview Video Coding (MVC) moof.

A stream in which samples of the left-view and right-view images of astereoscopic image are alternately arranged is illustrated in the upperpart of FIG. 12, whereas the stream is rearranged according to the leftand right views, for more effective indexing and access and thenrearranged according to temporal levels, as illustrated in the lowerpart of FIG. 12.

To describe one or more levels as stated above, subfragments are indexedaccording to viewpoints based on sidx_extension illustrated in Table 3and then temporal level-based index information about the subfragment isprovided by reserved_bit.

Application Example 2 (Sample Group Index Box Extension)

SampleGroupindexBox(sgix) provides the index of a sample group(subfragments) in a moof. As in the afore-described sidx_extensionmethod, type information and level information for each sample group(subfragments) are added.

Table 5 illustrates a syntax of SampleGroupindexBox added with priorityinformation, according to an embodiment of the present invention.

TABLE 5 aligned(8) class SampleGroupIndexBox   extends FullBox(‘sgix’,0, 0) {   unsigned int(32)  fragment_count;     unsinged int(16)level_count;     unsigned int(8) fragment_type;     for( i=0; i <fragment_count; i++) {       for ( j=1; j < level_count; j++) { unsignedint(8) level; unsigned int(32) offset; unsigned int(32) size;       }    } }

fragment_type: indicates the configuration method of a sample group(subfragments), identical to assemble_type in sidx_extension;

level: defines the level of a sample group (subfragments). A lower valueindicates a higher priority;

offset: the offset of a sample; and

size: the size of a sample.

Application Example 3 (sidx_extension)

Table 6 illustrates a syntax according to an sidx_extension-based methodaccording to an embodiment of the present invention.

TABLE 6 aligned(8) class SegmentIndexBox extends FullBox(‘sidx’,version, 0) {   unsigned int(32) reference_track_ID;   unsigned int(16)track_count;   unsigned int(16) reference_count;   for (i=1; i<=track_count; i++)   {     unsigned int(32) track_ID;     if (version==0)    {       unsigned int(32) decoding_time;     } else     {      unsigned int(64) decoding_time;     }   }   for(i=1; i <=reference_count; i++)   {     bit (1) reference_type;     unsignedint(31) reference_offset;     bit(1) contains_level;   //indication flagfor level, added     unsigned int(31) subsegment_duration; //reduce onebit     for contains_level, modified     bit(1) contains_RAP;    unsigned int(31) RAP_delta_time;     if (contains_level) //added    {       unsigend int(8) assemble_type; //define assemble type,      add       unsigned int(16) level_count; //number of level(e.g.      temporal id), added       for(i=1; i <= level_count; i++) //added      {         unsigned int(8)  level; //level(e.g. temporal id        level) define, added         bit(1) reserved_bit; //reserved        unsigned int(31) level_offset; //offest, added        if(assemble_type == 0x01 || assemble_type == 0x03)         {         unsigned int(16) offset_count //define number          ofsamples in level, added          for(j=1; j <= offset_count;j++) //added          {           unsigned int(32) offset; //offset from          level_offset, added           unsigned int(32) size; //size ofeach sample           in level, added          }         }       }     }  } }

Parameters in Table 6 have the following meanings

contains_level: a flag bit indicating whether subfragmentwise indexinformation is included; and

assemble_type: has an integer, as an indicator indicating the type of asample group, which may be formed according to its characteristics. Forinstance, samples can be grouped largely into a video sample group andan audio sample group.

Video samples may be divided into more sample groups according to theirpurposes. For instance, independently decodable samples to support TRAmay be grouped into one sample group. In case of a video supportingtemporal scalability, sample groups may be formed in such a manner thateach sample group includes samples with the same temporal level. In caseof a stereoscopic video, samples may be grouped according to viewpoints.MVC samples may also be grouped according to view ids.

level_count: the total number of levels in a fragment;

level: defines each level. A lower value indicates a higher priority.According to the meaning of level_count, level can be used differently.If a priority is assigned according to a higher or lower value oflevel_count, the level field is not included. If a level is assignedirrespective of level_count, this level field is particularlymeaningful;

level_offset: position information about each level;

offset_count: an offset count to support samplewise access, when needed;

offset: the position of a sample;

size: the size of a sample; and

reserved_bit: a reserved bit for extension.

Application Example 4 (Sample Group Index Box Extension)

SampleGroupindexBox(sgix) provides the index of a sample group(subfragments). As in the afore-described sidx_extension method, typeinformation, level information, and size information (SampleGroupSize)for each sample group (subfragments) are added.

Table 7 illustrates a syntax of SampleGroupindexBox added with priorityinformation, according to an embodiment of the present invention.

TABLE 7 aligned(8) class SampleGroupIndexBox   extends FullBox(‘sgix’,0, 0) {   unsigned int(32)  fragment_count;     unsinged int(16)level_count;     unsigned int(8) fragment_type;     for( i=0; i <fragment_count; i++) {       for ( j=1; j < level_count; j++) { unsignedint(8) level; unsigned int(32) SamepleGroupSize;       }     } }

Parameters of Table 7 have the following meanings

fragment_type: has an integer, as an indicator of the type or accesspoint of a sample group. Sample groups may be grouped largely into avideo sample group and an audio sample group according to media types.The video samples may be further classified into more detailed samplegroups. For instance, independently decodable samples to support TRA maybe grouped into the same sample group. In case of a video supportingtemporal scalability, sample groups may be formed in such a manner thateach sample group includes samples with the same temporal level. In caseof a stereoscopic video, samples may be grouped according to viewpoints.MVC samples may also be grouped into sample groups each having the sameviewpoint. If a scalable video supports one or more scales, one or moresample groups may be defined accordingly. fragment_type of a samplegroup may be described as the type or access point of the sample group;

level: defines each level. A lower value indicates a higher priority.According to the meaning of level_count, level can be used differently.If a priority is assigned according to a higher or lower value oflevel_count, level is not included. If a level is assigned irrespectiveof level_count, this level field is particularly meaningful. Table 8illustrates a syntax with the level field not included; and

SampleGroupSize: the size of a sample group. Since information about thesize and offset of each sample can be acquired from the sample group towhich the sample belongs, only the size of each sample group ispresented.

Table 8 illustrates a syntax of SampleGroupindexBox in which a samplegroup is indicated by fragment_type only, according to an embodiment ofthe present invention.

TABLE 8 aligned(8) class SampleGroupIndexBox   extends FullBox(‘sgix’,0, 0) {   unsigned int(32)   fragment_count;     unsigned int(16)level_count;     unsigned int(8) fragment_type;     for( i=0; i <fragment_count; i++) {       for ( j=1; j < level_count; j++) {        unsigned int(32)   SampleGroupSize;       }     } }

According to an embodiment of the present invention, TRA is supported asfollows.

To move to a specific time point requested by a user or play back at amultiple speed, information about a Segment corresponding to the timepoint is acquired from an MPD. Information about a moof corresponding tothe requested time point is checked using sidx of the Segment, andindependently decodable samples of the moof are accessed based on thedisclosed SampleGroupindexBox.

Level may also be used as a sample group identifier. If one or moresample groups should be correlated using an identifier, the same valuemay be assigned to the sample groups using the level field. Forinstance, if a stereoscopic video is classified into a left-view samplegroup and a right-view sample group, the same level value is assigned tothe sample groups. In this manner, a relationship between sample groupscan also be defined using, for example, the level field for SVC or MVC.

On the other hand, to describe the relationship between the samplegroups of a stereoscopic service without utilizing the level field,embodiments of the present invention introduce an additional new box.For example, when the disclosed box is used to support the TRA of thestereoscopic service, fragment_type and SampleGroupSize are provided. Tosupport this scheme, the temporal/spatial relationship between samplegroups should be indicated by dependency_id and a sample group ID.

Application Example 5 (Subfragment Index Box Extension)

SubFragmentIndexBox (sfix) provides index information about a sub-samplegroup or a subfragment as in Application Examples 2 and 4.SubFragmentIndexBox (sfix) is extended as illustrated in Table 9 inorder to support access to a subfragment or sub-sample group that is apart of a fragment due to such factors as a user request or a networkenvironment.

Table 9 illustrates a syntax of sfix extension to provide indexinformation about a subfragment or sub-sample group, according to anembodiment of the present invention.

TABLE 9 aligned(8) class SubFragmentIndexBox   extends FullBox(‘sfix’,0, flags) {       unsigned int(32)  fragment_count;       unsignedint(8)   fragment_level_count;       for ( j=1; j <fragment_level_count) {         if ((flags & 1) == 0)           unsignedint(32) track_id;         else           unsigned int(32) sub_track_id;      }       for( i=0; i < fragment_count; i++)         for ( j=0; j <fragment_level_count; j++)           unsigned int(32)accumulated_level_size; }

In Table 9, sfix provides index information about a subfragment orsub-sample group. In a scenario in which a part of a fragment iscombined or used, such as SVC, MVC, TRA situations, specifically when anindependently decodable subfragment or sub-sample group of a fragmentshould be accessed for TRA, a CODEC is adaptable as for SVC, or only asubfragment or sub-sample group with a specific scale is used, anindicator should be provided to enable access to the subfragment orsub-sample group. Accordingly, track_id or sub_track_id is provided tothereby access a specific subfragment of sub-sample group, asillustrated in Table 9.

Parameters and their meanings in Table 9 are given as follows.

fragment_count: the number of fragments included in sfix;

fragment_level_count: the number of levels included in each fragment;

track_id: indicates that a subfragment or sub-sample group includes apart of data in a track indicated by track_id;

sub_track_id: indicates that a subfragment or sub-sample group includesa part of data in a sub-track indicated by sub_track_id. Although idsare not assigned to sub-tracks at present, sub_track_id may be disclosedin the future. Otherwise, it may be preset that sub_track_id increasesgradually according to the number of sub-tracks included in a track,when sub_track_id is used; and

accumulated_level_size: the total size of a subfragment or sub-samplegroup.

The above sfix extension does not include configuration informationabout each subfragment or sub-sample group (fragment_type orassemble_type in Application Examples 1 to 4). Therefore, the type ofthe subfragment or sub-sample group may be indicated in a subfragmentbox or sub-sample group box.

The above-described methods according to embodiments of the presentinvention may be used in combination, when needed.

As is apparent from the above description of embodiments of the presentinvention, a transmission delay can be reduced significantly bydiscontinuing data transmission upon timeout. When a packet is lost dueto fading of a radio channel, ACK spoofing is used to thereby fullyutilize a bit rate. In addition, an initial screen is quickly invokedusing a preview method in a slow start environment, therebysignificantly reducing an initial buffering delay and a channel zappingdelay. Accordingly, a user can enjoy an HTTP streaming service moreconveniently, fully utilizing available radio resources.

While the present invention has been particularly shown and describedwith reference to embodiments thereof, it will be understood by those ofordinary skill in the art that various changes in form and details maybe made therein without departing from the spirit and scope of thepresent invention as defined by the following claims.

What is claimed is:
 1. A method for providing a multimedia streamingservice, comprising: transmitting, by a server, a Media PresentationDescription (MPD) including information about media data to a client;receiving from the client, by the server, a partial request messagerequesting a part of media data having a range according to a definedrange in the MPD; and transmitting to the client, by the server, asegment having the range in response to the partial request message,wherein the segment includes at least one fragment, segment indexinformation including information indicating a position of the at leastone fragment in the segment, and fragment index information includinginformation indicating a position of each of a plurality of samplesincluded in the at least one fragment, and wherein the fragment indexinformation includes information about a total size of the samples. 2.The method of claim 1, wherein the fragment index information furtherincludes information about the number of levels representing prioritiesof the samples.
 3. The method of claim 1, wherein the segment indexinformation includes an indicator indicating an arrangement method ofthe samples in the at least one fragment.
 4. A method for receiving amultimedia streaming service, comprising: receiving from a server, by aclient, a Media Presentation Description (MPD) including informationabout media data; detecting, by the client, segment information in thereceived MPD; transmitting to the server, by the client, a partialrequest message requesting a part of media data having a range accordingto a defined range in the MPD; and receiving from the server, by theclient, a segment having the range in response to the partial requestmessage, wherein the segment includes at least one fragment, segmentindex information including information indicating the position of theat least one fragment in the segment, and fragment index informationincluding information indicating the position of each of a plurality ofsamples included in the at least one fragment, and wherein the fragmentindex information includes information about a total size of thesamples.
 5. The method of claim 4, further comprising accessing aspecific sample using the fragment index information, and decoding thespecific sample.
 6. The method of claim 4, wherein the fragment indexinformation further includes information about the number of levelsrepresenting priorities of the samples.
 7. The method of claim 4,wherein the segment index information includes an indicator indicatingan arrangement method of the samples in the at least one fragment.
 8. Aserver apparatus for providing a multimedia streaming service,comprising: a Media Presentation Description (MPD) transmitter fortransmitting an MPD including information about media data to a client;and a transceiver for receiving, from the client, a partial requestmessage requesting a part of media data having a range according to adefined range in the MPD, and transmitting to the client a segmenthaving the range in response to the partial request message, wherein thesegment includes at least one fragment, segment index informationincluding information indicating the position of the at least onefragment in the segment, and fragment index information includinginformation indicating the position of each of a plurality of samplesincluded in the at least one fragment, and wherein the fragment indexinformation includes information about a total size of the samples. 9.The server apparatus of claim 8, wherein the fragment index informationfurther includes information about the number of levels representingpriorities of the samples.
 10. The server apparatus of claim 8, whereinthe segment index information includes an indicator indicating anarrangement method of the samples in the at least one fragment.
 11. Aclient apparatus for receiving a multimedia streaming service,comprising: a Media Presentation Description (MPD) receiver forreceiving, from a server, an MPD including information about media data;a data requester for detecting segment information in the received MPDand transmitting to the server a partial request message requesting apart of media data having a range according to a defined range in theMPD; and a data receiver for receiving, from the server, a segmenthaving the range in response to the partial request message, wherein thesegment includes at least one fragment, segment index informationincluding information indicating the position of the at least onefragment in the segment, and fragment index information includinginformation indicating the position of each of a plurality of samplesincluded in the at least one fragment, and wherein the fragment indexinformation includes information about a total size of the samples. 12.The client apparatus of claim 11, further comprising a decoder foraccessing a specific sample using the fragment index information anddecoding the specific sample.
 13. The client apparatus of claim 11,wherein the fragment index information further includes informationabout the number of levels representing priorities of the samples. 14.The client apparatus of claim 11, wherein the segment index informationincludes an indicator indicating an arrangement method of the samples inthe at least one fragment.